One of the biggest problems caused from vehicles using fossil fuel, such as gasoline and diesel oil, is creation of air pollution. A technology of using a secondary battery, which can be charged and discharged, as a power source for vehicles has attracted considerable attention as one method of solving the above-mentioned problem. As a result, electric vehicles (EV), which are operated using only a battery, and hybrid electric vehicles (HEV), which jointly use a battery and a conventional engine, have been developed. Some of the electric vehicles and the hybrid electric vehicles are now being commercially used. A nickel-metal hydride (Ni-MH) secondary battery has been mainly used as the power source for the electric vehicles (EV) and the hybrid electric vehicles (HEV). In recent years, however, the use of a lithium-ion secondary battery has been attempted.
High output and large capacity are needed for such a secondary battery to be used as the power source for the electric vehicles (EV) and the hybrid electric vehicles (HEV). For this reason, a plurality of small-sized secondary batteries (unit cells) are connected in series or in parallel with each other so as to construct a battery module, and a plurality of battery modules are connected in parallel or in series with each other so as to construct a battery pack.
In such a high-output, large-capacity secondary battery, however, a large amount of heat is generated from the unit cells during the charge and the discharge of the unit cells. When the heat generated from the unit cells during the charge and the discharge of the unit cells is not effectively removed, heat is accumulated in the unit cells with the result that the unit cells are degraded. Consequently, it is necessary to provide a cooling system for vehicle battery packs, which are high-output, large-capacity secondary batteries.
Generally, a cooling system for vehicle battery packs is constructed in an air-cooling structure using air as a coolant. In the air-cooling structure, air is introduced into the cooling system from the outside of the cooling system to cool the battery pack, and is then discharged out of the cooling system. The outside of the cooling system includes the inside of a vehicle, such as a trunk and a passenger space, as well as the outside of the vehicle. However, the cooling system with the above-described construction has several problems.
First, the air used as the coolant is supplied from the outside of vehicle with the result that the air is considerably affected by external conditions, such as humidity and temperature. For this reason, it is necessary to provide an additional device for controlling the temperature and the humidity of the air. As a result, the size of the cooling system is increased. When air inside the vehicle is used as the coolant, the above problem is somewhat solved. In this case, however, it is necessary to provide inlet and outlet ports for introducing and discharging air, respectively. As a result, the size of the cooling system is increased.
Second, when, in spite of removal of foreign matter by filtering the coolant, some of the foreign matter is introduced through the inlet port and the outlet port, and is brought into contact with unit cells of a battery pack, the outer surfaces of the unit cells may be physically or chemically damaged. In addition, noise and vibration generated due to the operation of a coolant introduction device, such as a fan or a pump, may be transmitted to a passenger(s) through the inlet port and the outlet port.
In order to solve the above-stated problems, Japanese Unexamined Patent Publication No. 2001-023703 and No. 2002-251951 disclose a sealed type housing for battery packs. In the disclosures of these publications, a coolant inlet port and a coolant outlet port are omitted, and heat is absorbed using a heat transfer plate or a wide cooling flow channel is provided between unit cells to remove heat from the unit cells, thereby solving the problems caused from the inlet port and the outlet port as described above. Nevertheless, a large amount of heat cannot be effectively removed by the structures disclosed in the publications. As a result, the structures disclosed in the publications are not suitable for vehicles using a large-sized battery pack, such as electric vehicles (EV) and hybrid electric vehicles (HEV).
Meanwhile, the vehicles, such as the electric vehicles (EV) and the hybrid electric vehicles (HEV), may be operated under the tough conditions. For example, when a vehicle is at low temperature in the winter season, it is necessary to stop the operation of a cooling system such that a battery pack can be operated at the optimum operating temperature. Alternatively, it may be necessary to increase the temperature of air introduced into the battery pack such that the operating temperature of the battery pack can be controlled to be the optimum temperature level. In this case, it is necessary to provide an additional device. In addition, when the unit cells of the battery pack have low temperature, battery components may be damaged. Furthermore, the degradation of the battery components may be accelerated due to the abrupt increase of the temperature of the air introduced into the battery pack.